1. Field of the Invention
The present invention relates to an image forming apparatus that controls a pressing force of a nip portion formed by a first rotation member for heating a recording medium having an image formed thereon and a second rotation member.
2. Description of the Related Art
There is widely used an image forming apparatus that transfers a toner image formed on an image bearing member to a recording material directly or via an intermediate transfer member, and heats and applies pressure on the recording medium to which the toner image has been transferred by a nip portion of a fixing device as an example of an image heating device to fix the image on the recording medium. The image heating device forms a nip portion of the recording medium by press-contacting a press contact rotation member (belt member or roller member) having an elastic layer with a heating rotation member (belt member or roller member). In the image heating device, to prevent deformation of the elastic layer, the press contact rotation member is desirably separated from the heating rotation member during stop period (as discussed in Japanese Patent Application Laid-Open No. 2005-114959).
Japanese Patent Application Laid-Open No. 2005-114959 discusses a mechanism for releasing the pressure of the nip portion applied by a spring member by rotating a cam member via a motor. The pressure of the nip portion is reduced or released during a period where heating processing of the recording medium is not performed.
Japanese Patent Application Laid-Open No. 2005-114959 discloses an image heating device that selects and executes a pressurization mode for performing fixing processing, a pressurization releasing mode for releasing pressure of a nip portion during stop period, and a pressurization reducing mode for setting image formation standby in a state where a pressurizing force is reduced.
The applicant has discovered that when the cam member receives acceleration in a speed increasing direction from the surrounding mechanism, a rotor rotates at a speed equal to or higher than a rotational speed of a rotating magnetic field of a stator, and then the cam member overruns when it is braked to stop.